The invention relates to sea-going vessels of the displacement (non-planing type) and in particular to the form of the hull for such vessels.
To date marine hulls have generally been designed on the principle that the hull is rounded to cause the minimum resistance to flow. This rounding has resulted in the classical clipper lines where the mid-body is faired into the bow and stern, with rounded bilges amidships (where the sides joint the bottom) and with compound curves at bow and stern.
In order to damp roll, bilge keels are added, and these carefully follow the flow lines, much effort being expended in tank towing trials to ensure that the keels do not exert avoidable drag.
On the other hand, some working vessels function mostly as stationary platforms, and are not designed for speed or efficiency in motion. Competing factors in the design are speed, efficiency, stability and cost. Other vessels, such as floating production, storage and off-loading (FPSO) vessels are designed to operate permanently at one location and may not even be designed to travel under their own power, but only with tug assistance.
Vessels required in the oil and gas production industry include drilling vessels, production vessels, well intervention vessels, accommodation vessels and the like. Such vessels typically have lengths (at the water line) in the range of 80 m-250 m (especially 100 m-200 m) and beam in the 15 m-30 m (especially 18 m-25 m). One known example used for various offshore support activities has a box shaped hull with ends shaped like a Thames lighter. The known vessel in fact bears a strong resemblance to a skip. The bilges are right angles and no bilge keels are fitted. This sharp edge or xe2x80x9cchinexe2x80x9d gives vastly improved roll damping. This, together with a highly sub-divided ballast system that allows optimum stability to be maintained for any load condition, gives the vessel a truly remarkable stillness in the water. The flat sides and bottom make it very cheap to build. Unfortunately, such a vessel is extremely slow (6 knots max.), making it unsuitable for use in a world-wide business.
In conventional design practice for sea-going vessels, it has generally been accepted that, no matter how much power is installed in a vessel of about 100-250 m with a rectangular cross section in the mid body, there is no possibility of pushing the vessel through the water at any speed above about 10 knots. The calculated power consumption is on the steep part of an exponential curve.
The present inventors had a desire to provide a new vessel having the stability and motion characteristics of the known vessel mentioned above, with a high transit speed. It would also be desirable to exploit the cheapness of construction afforded by avoiding as much as possible forming the hull of compound curves. Compound curves will be understood to refer to concave and convex surface portions having a substantial curvature in two dimensions, which require expensive pre-forming operations prior to assembly of the vessel. Those surface portions being part-conical, part cylindrical, or otherwise curved in one dimension only, together with twisted planar surfaces, will be referred to herein as portions having simple curves.
Of course, various forms of hull have been known for water-borne use, each form having evolved for a different type of use and/or environment. It was observed that in Rhine barge practice, the tug power is far below the theoretical requirements for such a hull, as indicated by conventional marine theory. In the case of Mississippi barges, the inventors found the same mismatch with conventional marine theory. Unfortunately, such barges would not survive long in the open sea.
The invention in a first aspect provides a hull for a sea-going vessel of displacement type having a flat bottomed midship section of a constant cross-section which is substantially rectangular in cross section below the water line, and a converging bow portion extending from said midship section, said bow portion having a curved transition between the bottom thereof and each hull side, the transition between bottom and sides along the bilges in said midship section being relatively sharp, and in particular of radius less than 0.5 m.
The bow shape may include a substantially vertical part-cylinder topped by a flared prow, for example, a part cone, with substantially simple curved and planar sheets leading back into the body, avoiding compound curves for the major part of the bow section. Compound curves may be employed judiciously to improve speed, but need only represent a few percent of the bow section surface area below the water line, less than thirty or twenty percent, for example.
The vertical part cylinder forming the stem of the vessel below the water line may be extremely fine, effectively resulting in a pointed bow. Alternatively, given a beam of say 20 m, the stem may have a radius at the water line up to 0.5, 0.75 or even 1 m about the vertical axis. The part cylinder may be replaced by a part cone, having a finer radius or even a sharp point at the forefoot (where the stem meets the ships""s bottom), and a larger radius where it meets the more flared prow above the water line.
The same cylindrical portion may curve aftwards to join the bottom of the vessel, forming a minor forefoot portion of compound curve.
The flared prow above the water line affords a marked, steady increase in available buoyancy in waves beyond a certain size. The same principle may be applied to a stern that divides a following sea and flares to provide the required increase in buoyancy without slamming.
Alternatively, in accordance with a second aspect of the invention, there is provided a sea-going vessel with (at least below the water line) a substantially rectangular midship section, in which the stern shape includes at least one propulsion arch (preferably two) formed by an inclined, substantially part-cylindrical surface extending from the flat bottom aft and upward to join the transom.
In a preferred embodiment, suitable for a dynamic positioning (DP) vessel, there are a pair of inclined part-cylindrical propulsion arches, the hull the arches being flared together so as to facilitate transverse thrusting from a steerable propulsion unit mounted in each arch. The flaring may be formed by a pair of twisted flat surface portions.
In accordance with a third aspect of the invention, a sea-going vessel having a flat bottomed midship section has a stern section provided with propulsion arches formed such that water flowing to occupy the propulsion arch comes predominantly from beneath the bottom, rather than from the sides of the vessel as it moves through the water. This is believed to be an advantageous and novel feature in sea-going vessels, as opposed to barges. Such behaviour may be achieved by the reclining part-cylinder construction mentioned above. In profile, the arches and associated portions may provide a marked flare, so as to provide reserve buoyancy to counteract pitching and to xe2x80x9crollxe2x80x9d waves approaching from the quarters of the vessel.
The transom may be flat. Inclined intermediate flat portions may be provided between the curved arch and the transom to reduce drag. A transition zone of compound curve shape may be provided between the arch portions and the flat bottom portion, again to reduce separation in the water flow and consequent drag.
A fourth aspect of the invention provides a sea-going vessel, the hull of the vessel below the water line comprising less than five percent compound curves in surface area.
A fifth aspect of the invention provides a sea-going vessel, the hull of the vessel below the water line comprising a parallel midship section and bow and stern sections, the bow section comprising less than thirty, optionally less than twenty or ten percent compound curves in surface area.
A sixth aspect of the invention provides a sea-going vessel, the hull of the vessel below the water line comprising a parallel midship section and bow and stern sections, the stern section comprising less than thirty, optionally less than twenty or ten percent compound curves in surface area
In any of these aspects the vessel may be over 80 m in length at the water line and over 15 m beam. It may have a maximum speed in excess of 10 knots. The vessel may be equipped for sub-sea operations, by the provision of cranes, dredging, diving, pipe- or cable laying equipment, and/or by the provision of one or more vertical openings (moonpools) in the vessel bottom. The vessel may in particular be equipped with various thrusters and computer control for dynamic positioning (DP). The vessel may have a variable deck load capacity in excess of 800 tonnes.
The vessel may have a substantially rectangular midship cross section with bilges (chines) of radius less than 0.5 m or optionally less than 0.2 m or 0.1 m or 0.05 m. The bridge at the junction of flat sides and bottom may in particular be constructed entirely without curved plates and without a bilge keel. In a preferred embodiment, the side and bottom plates are welded together via a solid bar or pipe of small diameter. The bar may be of rectangular or round cross section. It may have a diameter less than five times the thickness of the side and bottom plates. Using a 50 mm bar, for example, the radius curvature at the bilge may for example be just 25 mm.
The invention further provides a hull, which may be fabricated according to any of the above aspects at one location, for subsequent fitting at another location.